Earth-boring tools for forming wellbores in subterranean earth formations may include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Similarly, roller cone earth-boring rotary drill bits may include cones that are mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit.
The cutting elements used in such earth-boring tools often include polycrystalline diamond cutters (often referred to as “PDCs”), which are cutting elements that include a polycrystalline diamond (PCD) material. Such polycrystalline diamond cutting elements are formed by sintering and bonding together relatively small diamond grains or crystals under conditions of high temperature and high pressure in the presence of a catalyst (such as, for example, cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as high temperature/high pressure (or “HTHP”) processes. The cutting element substrate may comprise a cermet material (i.e., a ceramic-metal composite material) such as, for example, cobalt-cemented tungsten carbide. In such instances, the cobalt (or other catalyst material) in the cutting element substrate may be drawn into the diamond grains or crystals during sintering and serve as a catalyst material for forming a diamond table from the diamond grains or crystals. In other methods, powdered catalyst material may be mixed with the diamond grains or crystals prior to sintering the grains or crystals together in an HTHP process.
PDC cutting elements commonly have a planar, disc-shaped diamond table on an end surface of a cylindrical cemented carbide substrate. Such a PDC cutting element may be mounted to an earth-boring rotary drag bit or other tool using fixed PDC cutting elements in a position and orientation that causes a peripheral edge of the diamond table to scrape against and shear away the surface of the formation being cut as the drill bit is rotated within a wellbore. As the PDC cutting element wears, a so-called “wear scar” or “wear flat” develops that comprises a generally flat surface of the cutting element that ultimately may extend from a front, exposed major surface of the diamond table to a cylindrical lateral side surface of the cemented carbide substrate.
Early PDC cutting elements had relatively thinner diamond tables having an average thickness of about one (1) millimeter or less. As such cutting elements were used to cut formation material, the wear scar that developed often included an uneven profile wherein the surface of the diamond table that was rubbing against the formation projected outward from the cutting element beyond the adjacent surface of the cemented carbide substrate that was rubbing against the formation. It was believed that this phenomenon was due to the fact that the rubbing surface of the cemented carbide substrate was wearing at a faster rate than was the rubbing surface of the diamond table. The portion of the diamond table at the wear scar projecting outward beyond the adjacent rubbing surface of the cemented carbide substrate has been referred to as a “shear lip.” The formation of such a shear lip was thought to beneficially result in an increased rate of penetration (ROP), although the shear lip was also frequently believed to be the source of delamination or spalling of the diamond table, which often leads to catastrophic failure of the cutting element.
Due at least partially to improvements in methods of forming polycrystalline diamond tables, PDC cutting elements are commonly fabricated with relatively thicker diamond tables having thicknesses of about four (4) millimeters or more. It has been observed that a shear lip does not often faun at the wear scar of such PDC cutting elements when used to cut formation material. Furthermore, as a PDC cutting element wears during use, the total area of the wear scar gradually increases. With PDC cutting elements having relatively thicker diamond tables, the total diamond surface area at the wear scar can reach a magnitude that results in a relatively slow ROP, as the large diamond surface area acts as a bearing surface upon which the cutting element rides across the formation, spreading the applied weight on bit over an unduly large surface area and hindering penetration of the cutting edge of the cutting element into the formation material.